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Galvanic corrosion of metals:

This article defines galvanic corrosion and explains the galvanic scale, the effects of corrosion on metal roofing, and an explanation of the galvanic scale and causes of corrosion between dissimilar metals in any application.

Here we explain the galvanic scale, the effect of corrosion caused when certain metals are placed in contact, and we provide examples of galvanic corrosion hazards that occur in buildings metal roofing, building electrical components, building plumbing components, and at underground oil storage tanks and oil piping systems.

Links at the end of this article provide further detail about rust and corrosion on nearly every building component where corrosion is a particular concern.

With metal roofing or any metal building components, the
safest strategy is not to mix metals that come in direct contact
with one another. Use aluminum flashing and fasteners
with aluminum roofing, copper flashing and copper nails
with copper roofing, etc. When this is not possible, choose
a second metal that is not likely to lead to galvanic corrosion
or use a physical barrier to separate the two metals.

What is the Galvanic Scale?

The galvanic scale (see Table 2-11 at left) ranks a metal’s tendency to react in contact with another
metal in the presence of an electrolyte, such as water or
even moisture from the air.

[Click any image or drawing to see a larger copy]

Metals at the top of the chart
are called anodic, or active, and are prone to corrode;
metals at the bottom are cathodic, or passive, and rarely
corrode.

The farther apart two metals are on the chart, the
greater their tendency to react and cause corrosion in
the more active metal. Metals close to each other on the
scale are usually safe to use together.

The Area Effect Determines the Rate of Metal Corrosion

The rate of corrosion is controlled by
the area of the more passive metal. For example, a galvanized
steel nail (active) will corrode quickly if surrounded
by a large area of copper flashing (passive). If a copper
nail is used in galvanized steel flashing, however, the corrosion
of the steel will be slow and spread over a large
area, so it may not be noticeable. In each case, the active
metal corrodes, and the passive metal is protected.

Galvanic Corrosion of Metal Roofing

Because they are
made from active metals, aluminum and zinc roofing
panels, as well as steel roofing with aluminum and zinc
coatings (galvanized steel, Galvalume®, etc.), are vulnerable
to galvanic corrosion if allowed to come in contact
with more passive metals. [Click any image or drawing to see a larger copy]

For example, never use copper
or lead flashings with aluminum, zinc, or galvanized roofing
materials. Even water dripping from a copper pipe,
flashing, or gutter can lead to corrosion of coated-steel or
aluminum roofing materials.

How common flashing materials
react with metal roofing and other metal building
materials is shown in Table 2-12 above.

Where incompatible metals must be used in close
proximity, use the following precautions:

Separate the two dissimilar metals with building
paper, bituminous membrane, durable tapes, or
sealants so they are not in direct contact.

Coat the cathodic (less active) metal with a nonconductive
paint or bituminous coating.

Other Incompatible Materials Found on Metal Roofs

In addition to galvanic corrosion, a number of other common
building materials can harm the finishes on metal roofing
or lead to etching or corrosion of the material itself:

Wet Mortar Effects on Metal Roofing

Aluminum roofing materials and aluminum based
coatings can be damaged by alkali solutions such as
wet mortar. Where contact with wet mortar cannot be
avoided, one option is to spray the metal with lacquer or a
clear acrylic coating to protect it until the mortar is dry.

Pressure-Treated Wood Effects on Metal Roofing

Roof panels treated with
aluminum and zinc coatings should not come into direct
contact with pressure-treated (PT) wood, which can damage
the finish and accelerate corrosion.

Sealants & Caulks Impact on Metal Roofs

Use only sealants recommended by the manufacturer.
Never use acid-cure silicones (the most common
type, with a vinegar smell) or asphalt roofing cement with
coated-steel roofing, as these will mar the finish. Commonly
recommended products include butyl tape and
gunnable terpolymer butyl or urethane sealant.

Salt Spray Impact on Metal Roofs

Saltwater spray is very hard on metallic coated–
steel products and may lead to corrosion within
5 to 7 years. In these areas, the best choices are copper,
stainless steel, or painted aluminum. Hylar/Kynar® finishes
hold up best.

Other Examples of Corrosion Between Dissimilar Metals and the Need for Dielectric Fittings in buildings

Corrosion Protection for Electrical Panels, Wiring, & Grounding

Corroded copper grounding wires can also be unreliable as our photo shows. The copper wire was bonded to a galvanized-iron water pipe where corrosion was exacerbated by the combination of dissimilar metals and wet conditions.

At ELECTRICAL GROUND PIPE CORROSION we describe how stray voltage into the ground system can cause plumbing leaks or even damage to air conditioners and heat pumps.

Also see ALUMINUM WIRING HAZARDS where corrosion may be a factor in the reliability of some aluminum wiring connections, particularly in damp or wet locations and where aluminum is joined to copper without using the appropriate connectors and antioxidants.

Corrosion in electrical components, possibly including galvanic effects can cause more subtle hazards such as poor connections inside of electrical panels, switches, and junction boxes.

Lack of corrosion resistance of certain internal parts is considered to be a factor in the failure of the circuit
breakers.

At above-left we illustrate an uninsulated aluminum grounding conductor that corroded through where it contacted a masonry block wall.

Galvanized to Copper Pipe Connections - Use a Dieelectric Fitting to Avoid Corrosion

When connecting iron or galvanized iron pipes to copper in buildings, often corrosion and leaks will occur at the meeting of these two dissimilar metals.

Using a brass fitting to connect these two metals, or more commonly, using plastic or bronze fittings at the joint between these two metals will avoid future corrosion and leaks.

The photo (left) shows a galvanized iron union used to connect copper to galvanized iron. In the upper image you can just make out the black bronze ring built into this plumbing connector to avoid corrosion where the copper presses against the galvanized iron.

How do we explain that in some buildings we see direct copper-to-iron pipe connections with no corrosion? Luck? Maybe. But the corrosivity of the water is probably a factor in how rapidly copper-to-galvanized pipe connections will corrode and leak.

Spelling note that may help some searches: it's not dialectic pipe fittings, but dielectric pipe fittings.

Steel Underground Storage Tanks, Oil Piping, and Galvanic Corrosion

Dielectric material means a material
that does not conduct direct electrical
current. Dielectric coatings are used to
electrically isolate UST systems from the surrounding soils. Dielectric bushings
are used to electrically isolate
portions of the UST system (e.g., tank
from piping).

At OIL TANK FAILURE CAUSES we provide details about sources of corrosion in underground oil storage tanks and in their piping & connections.

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"The Fight Against Corrosion - A Study of the Nature of Corrosion and its Problems in Water Services and Heating Systems", Daniel Davies, Research and Development Services, Stansted Mountfichet, Essex, England, World Plumbing Conference-IV, "Plumbing and the World Environment, Compendium of Workshop Papers, October 3-6, 1996, Hyatt Regency Chicago, Chicago, IL", [personal correspondence, DJF - Author, July 2011]

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)

Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237,If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.